1. Field of the Invention
The invention relates to a cutting method by a wire saw and a cut workpiece receiving member in a wire saw and, more particularly, to a method for separating cut workpieces one from another during a workpiece cutting operation by a wire saw and a receiving member for favorably receiving the cut workpieces in separation one from another.
2. Description of the Related Art
In a wire saw, a wire reeled out from a feed side real is introduced to a tension mechanism and a load cell for measuring tension via a traverser and a guide pulley; and then, is wound numerous times around at least two work rollers, thereby forming a wire array; and finally, is wound around a retrieving side reel.
The above-described wire at the wire array across the work rollers is made to reciprocate or travel in one direction at a high speed, so that the workpiece is cut out into numerous wafers at the same time in abutment of the workpiece against the wire array while supplying, to the wire array, a slurry liquid containing a hard abrasive grain made of SiC or diamond or a water-soluble slurry liquid for the purpose of easy cleansing. Incidentally, a wire having diamond grains secured thereto via a resin or the like may be used.
Although a demand for cutting a workpiece such as, in particular, a silicon ingot for a solar cell has been increased in recent years in cutting the workpiece by the wire saw, as described above, a wafer thickness of 200 μm or less has been required for the purpose of cost reduction, thereby spurring more thinness.
As described above, as the thickness immediately after the cutting operation becomes smaller, it becomes difficult to handle a wafer after the cutting operation, thereby raising a problem of reduction of yield due to a crack of a wafer till a next cleansing process.
In a conventional workpiece cutting method by a wire saw, a workpiece B made of silicon or an ingot is adhesively secured to a workpiece machining mount 51 via a dummy member 52 constituted of a carbon or glass plate, as shown in FIG. 9A, and then, the workpiece B is pressed toward a wire A at a wire array, to be thus cut down to the dummy member 52, as shown in FIG. 9B. As a consequence, the workpiece B is cut in a comb-like state, so that wafers b are cut out.
Therefore, the wafers b cut out by the wire saw are preliminarily cleansed in the comb-like state, in which the wafers b are adhesively bonded to the dummy member 52 used in cutting, thereby removing the slurry liquid (i.e., a grinding liquid) 53. Subsequently, the bonded portions between the wafers b and the dummy member 52 are peeled off or are manually peeled off one by one, so that the wafers b are separated one from another.
However, a clearance equivalent to the diameter of the wire being, for example, 160 μm or lately 80 μm is undesirably formed between the wafers in the method for cutting the workpiece in the comb-like state. The slurry liquid stays in the clearance, and then, the surface tension of the slurry liquid narrows the clearance. Therefore, an outermost one out of the wafers b cut out in a thickness of about 100 μm is markedly curved and flexed, as shown in FIG. 9B, thereby raising a problem that the wafer weighted down with the slurry liquid 53 breaks and falls even with the application of a slight vibration.
In addition, since the cut-out wafers b remain bonded to the dummy member 52 in the method for cutting the workpiece into the comb-like shape, the workpiece is lifted up after the cutting operation, to be thus required to be drawn from the wire A. At this time, there arises a problem of the contact of the cut surface of the wafer with the wire, thereby exerting a flaw at the cut surface of the wafer.
In order to solve the above-described problems caused by cutting the workpiece into the comb-like shape, there has been proposed a cutting method for separating a plurality of wafers one from another, wherein a workpiece is fixed to a workpiece fixing table for fixing the workpiece with a space defined between facing surfaces of a dummy member secured to the workpiece fixing table and another dummy member secured to the workpiece, the dummy member on the workpiece side also is cut down to the space together with the workpiece at the same time, the wafer is separated from the dummy member on the workpiece fixing table side by the effect of the space upon completion of the cutting operation, and then, the wafer falling down by the cutting operation is received by a workpiece receiver (for example, see Japanese Patent Application Laid-open (JP-A) No. 2000-326324).
In the method in which the wafer falling down upon completion of the cutting operation is received by the workpiece receiver, a wire is positioned at a terminal end of the workpiece while a cutting start end of the workpiece becomes free as the wafer cutting operation approaches the final stage, so that the cutting start ends of the wafer are brought into contact with each other by the surface tension of a slurry liquid.
If the cutting operation proceeds in the above-described state, in particular, by using a wire saw of a down-cut type for cutting the wafer by pressing the workpiece from above a wire array, the cut and separated wafer is accidentally brought again into contact with an adjacent wafer during the falling caused by the surface tension of the slurry liquid. The plurality of wafers in contact with each other may be cracked even with the application of a slight vibration during the cutting operation owing to its own weight and the weight of the slurry liquid. Otherwise, the numerous wafers may be cracked if they fall at one time. As a consequence, there arises a problem of marked degradation of yield.
Furthermore, the contact between the ends of the wafers during the cutting operation undesirably flexes the wafer during the cutting operation, thereby degrading cutting accuracy. Moreover, the falling wafer leaning against the adjacent wafer may crack or deform the adjacent wafer by the weight of the leaning wafer.
Additionally, the surface tension of the slurry liquid need be broken in order to cleanse the wafers adhering to each other by the surface tension. Therefore, a considerably strong cleansing force is required, thereby arising a problem of the fracture of the wafer. Alternatively, the requirement of the strong cleansing force raises a problem from the viewpoint of environment or a cost. Furthermore, since the wafer is reduced in thickness, the wafer may be broken during a manually peeling operation, in particular.
Moreover, when the space is defined between the dummy members in fixing the workpiece to the workpiece fixing table, the workpiece is vibrated during the cutting operation by the use of the wire, thereby making it difficult to accurately cut the wafer.